1. Technical Field:
This invention relates to a method and apparatus for the production of high density monolithic, ceramic material, comprising subjecting elemental powders to a combustion synthesis process in a novel system to produce a porous material, and compacting the ceramic to high density by pressure wave initiated by a high explosive.
2. Background Information:
High-technology, structural ceramics are becoming utilized in ever larger range of applications which require light-weight, high-temperature, high-performance materials. These ceramics are typically the borides, carbides, nitrides and oxides of a variety of metals, and are fabricated in such a way as to eliminate most, if not all, porosity and impurities from the bulk. The production of these ceramics, however, involves complicated processes which, in general, use expensive starting materials, are very energy and labor intensive and so result in very high final product costs.
The most commonly used commercial process for fabricating these ceramics is hot pressing of the ceramic powder into the shape of a disc, rod or plate. Green (unfired) compacts of previously processed ceramic powders are placed into a die in a high temperature furnace and pressed with uniaxial pressure under inert gas atmosphere. While the sample is at high temperature, but well below the melting point of the ceramic, sintering (or solid welding) takes place during which the powder particles coalesce into a solid body. The length of time required for the whole body to be sintered may vary from a few to tens of hours. Since temperatures of 1200.degree. C. to 1600.degree. C. are quite common, these hot pressing operations can be very expensive from the energy use standpoint. In addition, the high temperature presses with atmospheric control represent a very high capital cost which increases in proportion to the volume of the sample to be processed.
Another means by which high-technology ceramics are fabricated is by Hot Isostatic Pressing (HIP). The major difference between HIP and hot pressing is that in HIP, isostatic pressure is exerted on the ceramic body during the high temperature cycle rather than uniaxial pressure. The sample to be fabricated is enclosed in a metal envelope, usually of tantalum or stainless steel, cold pressed to the desired shape, and then heated to temperatures up to 2000.degree. C. under the pressure of a working fluid, usually an inert gas, at a pressure in excess of 120 MPa. The advantage of HIP over hot pressing is that complicated shapes can be produced. In HIP, as in hot pressing, sintering is the mechanism by which sample consolidation takes place. However, the equipment and green compact preparation are even more complicated thus raising the product cost higher.
High-technology ceramic materials can also be produced by a process called Self-Propagating High-Temperature Synthesis (SHS). This process involves solid combustion reactions between constituent powders, which are characterized by very high heats of reaction, i.e., reaction temperatures of about 3000.degree. C., and reaction front velocities of a few cm/sec. It is possible to ignite the powder mixtures with a very small amount of energy at which point the heat of reaction that is released sustains further reaction until the whole sample has been synthesized. Fabrication by SHS has a number of distinct advantages over the conventional processes previously discussed. The fact that it is a high temperature process produces a self-purging effect, whereby most contaminants are driven from the sample during the reaction. Since all heat except for the small amount needed for ignition is supplied by the exothermic reaction, the process is highly energy efficient. In addition, the process is potentially more economical than the conventional processes since no high temperature furnace is needed. Because the product is formed at a temperature usually exceeding 2000.degree. C., phases and compositions that cannot be formed at the lower temperatures of conventional processing may be feasible.
The SHS process has been successfully used in a number of applications, both in the U.S. and abroad. Possibly the greatest successes have been achieved in the Soviet Union where the manufacturing of ceramic powders such at TiC, TiB.sub.2, SiC and B.sub.4 N, among others, is now done commercially. In addition, the Soviets are using SHS to produce tool bits, dielectric materials, heater elements and high temperature filters. In Japan, as well as the U.S., the production of materials by the SHS process has not progressed to the commercial stage as of yet, but significant applications are being pursued. In the U.S., applications have been limited to the use of SHS as a source of heat in thermal batteries and aerosol dispersal as a source of the IR signal in TOW missiles. The thermite reactions that are widely used for field welding of steel are probably the most common application of the SHS reaction principle.
However, there have been serious technical problems associated with the fabrication of high density products by this method. The first is the fact that when mixed powders are reacted by SHS, the product generally exhibits as much as 50% porosity, whereas as little as 0.5% porosity in ceramic materials can be detrimental to performance. The second problem is the cracking of the sample during processing. Gases formed from inpurities on the powders which when driven off at the high temperatures, form channels in the sample which become crack initiation sites. Cracking is also caused by the thermal shock to the sample as it cools down from over 2000.degree. C. to ambient in a very short time. If the sample is mechanically loaded in order to increase its density, and its temperature at the time of loading is below the ductile-brittle transition temperature, the internal stresses introduced during such loading may initiate cracking. Another problem is related to the bonding of final product grains to each other. The sample performance depends not only on the absence of porosity but also on the integrity of the inter-granular bonds. The long times at high temperatures needed for sintering action to take place (and its concomitant strong intergranular bonding) is not available for the SHS process since sintering temperatures are sustained for only a few minutes. The final problem is the difficulty in predicting the product properties and synthesis process behavior from the initial powder and compact properties, the initial geometry and ignition parameters.
Several experiments which utilize the SHS principle to achieve high density products of TiC and TiB.sub.2 have been attempted in U.S. laboratories. Titanium/Boron and Titanium/Carbon powder mixtures have been heated to ignition inside graphite dies under uniaxial pressure placed inside high temperature furnaces. Densities of about 95% have been achieved for the reacted product. The Ti/B and Ti/C powder mixtures placed inside insulated steel dies have been ignited by tungsten filaments or other small energy sources and reacted. After reaction, the porous product has been compacted by uniaxial pressing in a hydraulic press resulting in densities of about 88%. Ti/C mixtures, encased in insulated tubes have been ignited and continuously compacted in a rolling mill immediately following the passage of the reaction front. Small areas of high density have been achieved by this method.
Explosive consolidation technology has also been used in ceramics processing. An explosively generated shock wave has been used to attempt both ignition and compaction of Ti/B powder mixtures held in strong containment. The results of this experiment have shown complete SHS reaction of the powders but little or no compaction of the product TiB.sub.2.
This explosives technology has been applied to powdered metals resulting in successful consolidation of both cylindrical and plate forms. Factors affecting the consolidation are the pressure attained, load duration, and the material being compacted. A cylindrically converging system has been used almost invariably to consolidate high melting point ceramic powders. Starting with Al.sub.2 O.sub.3 powders at room temperature, explosively driven cylindrical compactions have produced material with reasonably pore-free local regions. However, the degree of compaction varies with radius and is sometimes further disrupted by thin spiraled regions of micro-cracked material that occurs because the compaction is spatially nonuniform.
The generally observed result is that large, crack-free specimens of hard, high melting point ceramics cannot be prepared by explosively compacting ceramic powders from the room temperature state. Aluminum nitride is an exception which is readily consolidated because, it is believed, it becomes plastic at high pressure. Partially successful explosive consolidation of ceramic powders has been accomplished by preheating the powders before compaction. Both cylindrical and flat plate samples, crack free and of high density have been made in this way.